GB960173A - Improvements in or relating to plastic film manufacture - Google Patents

Abstract

960,173. Infra-red emission measurements. IMPERIAL CHEMICAL INDUSTRIES Ltd. Oct. 24, 1962 [Nov. 1, 1961 ], No. 39849/61. Heading G1A. The temperature of aplastic film which requires heating during its manufacture is measured by measuring the infra-red emission from the film over a range. or group of wavelengths of strong emission by means of a measuring instrument which is sensitive only to those wavelengths. By restricting the wavelength range for the measurement, interfering radiation can largely be excluded from the measurement. Furthermore, by restricting the measurement to the range of strong emission (corresponding to the range of strong absorption) the film behaves substantially as a black body radiator and may function as a filter to remove intefering radiation. Thus, in a preferred method of making the measurement, the measuring instrument is placed on the opposite side of the film to the source of radiation employed for heating during manufacture. The wavelength range over which measurement is made is at least 0.1 micron and less than 5 microns in extent. Fig. 3 illustrates an arrangement for measuring the temperature of a film 11 of polyethylene terephthalate as it is passed through a stenter where it is stretched transversely by clips 14 and heat set. Radiation 15 from a point A where the stretching ceases and the heat setting commences is directed through a lens 15 to infrared detector comprising a pair of gas cells 19, 20 the pressure difference between which displaces a thin aluminium diaphragm 22 with respect to a thin aluminium mesh 21 to produce a variation in the capacitance formed therebetween. The cells are filled with nitrous oxide which has a strong absorption band at about 8 microns coinciding with a strong emission band from polyethylene terephthalate. As illustrated, the right-hand cell is exposed to the radiation 15 whilst the left-hand cell is shielded from radiation. In an alternative arrangement, the left-hand cell may be exposed through an adjustable wedge to a standard temperature comparison source. The radiation path to the detector includes a filter 17 comprising a layer of indium antimonide and a layer of mica which restricts the radiation reaching the detector to the range 7-9 microns. A rotary chopper 17 operating at 13 cycles per second is also included so as to cause the detector to produce an alternating current output signal which may be amplified by a tuned amplifier. The detector assembly is disposed on the side of the film 11 opposite to the source of heating radiation, a stop 25 being provided after lens 16 to absorb any radiation which is not absorbed by the film. The whole assembly is mounted in a thermostatically controlled enclosure 24. The detector may be moved across or along the film so as to permit the determination of the temperature distribution and the output signal may be employed for the automatic control of the temperature at various part of the stenter oven. Fig. 2 (not shown) illustrates an arrangement where the film is stretched longitudinally. Reference is made to the use of a detector including a aliphatic hydrocarbon gas for measurements on polypropylene film Fig. 4 illustrates another measuring device in which the radiation 32 from the film 31 is imaged by a lens 33 on an infra-red detector 39, the radiation path including a stationary filter 35 which transmits only over a range including the strong emission band or bands of the film and rotary chopper 34 constructed as shown in Fig. 5 of segments 42 of material which either has absorption characteristics identical to those of the film whose temperature is being measured or at least one infra-red absorption band in coincidence with a strong emission band of the film, the band being within the range defined by filter 35. For example, the material may be a sample of the film being measured. Alternatively the material may merely be similar chemically, e.g. a sample of the film which has not been orientated. Dependent upon the wavelength range used in the measurement, the detector 39 may comprise a thermopile, a photoemissive cell, a photoconductive cell or gas cell as in Fig. 3. The output of the cell, which is an alternating current signal at the "chopping" frequency imparted by chopper 34, is applied through a tuned amplifier 37 to a display instrument 38A stop 39 is provided to absorb any infra-red radiation which is transmitted through the film from a heater 40. The Specification includes a list of plastic materials to which the invention may be employed.